US President Donald Trump has threatened the total destruction of North Korea if it does not abandon its drive toward nuclear weapons.

In a speech to the United Nations general assembly, he said "it is far past time for the nations of the world to confront" North Korean dictator Kim Jong Un.

And he said Mr Kim's "reckless pursuit of nuclear weapons" poses a threat to "the entire world with an unthinkable loss of human life".

Referring to Mr Kim, the US President said: "Rocket man is on a suicide mission for himself and his regime."

And he said of the US: "If it is forced to defend itself or its allies, we will have no choice but to totally destroy North Korea."

North Korean leader Kim Jong Un

Observers said the speech prompted 'murmuring' from the UN audience.

Mr Trump, who has previously warned of "fire and fury" if the North Korean does not back down, claimed "no one has shown more contempt for other nations and for the wellbeing of their own people than the depraved regime in North Korea".

And he scolded nations that he said have enabled and traded with North Korea, seeming to slight China, though he did not mention it by name.

In addition to North Korea, Mr Trump urged nations to join together to stop Iran's nuclear programme.

He declared the Iran nuclear deal an "embarrassment" for the United States.

He also condemned the "loser terrorists" who have committed violence across the globe.

North Korean leader Kim Jong-un (Image: AP Photo/KRT via AP Video)

"I will always put American first. Just like you, the leaders of your countries, should and always put your countries first," said Mr Trump, who assured the UN that the US would not abdicate its leadership position in the world but needed other countries to contribute more.

"The US will forever be a great friend to the world and especially to its allies," the Republican president said.

"But we can no longer be taken advantage of or enter into a one-sided deal in which the United States gets nothing in return."